New paleomagnetic results from Neogene to Quaternary volcanic rocks of north of the Lake Van, Eastern Turkey

The Eastern Anatolia is an active tectonic region where the collision between the Arabian and Eurasian plates take place. Due to the subduction of Arabian plate’s oceanic lithosphere under Eurasian plate, widespread volcanism observed in large areas began in Serravallian. There is no consensus in the literature for the tectonic evolution of the region. Therefore, there are many geological and geophysical studies conducted with the intention of explaining the tectonic evolution of Eastern Anatolia by geodynamic models. Our paleomagnetism study aims to reveal the tectonic rotations in order to better understand the development of the prevailing tectonism in the region from the volcanic rocks. Paleomagnetic samples were collected from 86 sites of the Late Miocene–Pleistocene volcanic rocks located at the north of Lake Van. Isothermal remanent magnetization studies show that magnetite is the mineral responsible for magnetization in most rocks, while both magnetite and hematite are responsible for the rest of the rocks. Curie temperatures and alteration degrees of rock samples were also determined by high-temperature susceptibility (HTS) studies. In some samples, titanomagnetite component was observed in the heating phase of the HTS measurements. The absence of this component in the cooling step indicates that Ti-magnetite is transformed into magnetite by alteration. The Pleistocene volcanics show counterclockwise rotation of R ± ΔR = 13.4° ± 3.8°. The Pliocene volcanic rocks were defined in four different groups: south of Erciş Fault, north of Erciş Fault, around Muradiye and north of Van. Also, the remarkable clockwise rotation is observed in the north of Van and near Muradiye R ± ΔR = 24.4° ± 17.0° and R ± ΔR = 6.9° ± 9.4°, respectively. In addition, counterclockwise rotation (R ± ΔR = 14.5° ± 6.1°) is obtained in the southern part of the Erciş Fault, while there is no significant rotation (R ± ΔR = 0.6° ± 7.4°) on the northern side. Late Miocene volcanic rocks show no significant rotation either (R ± ΔR = 1.8° ± 13.7°). Our new paleomagnetic results indicate that the left-lateral strike-slip Çakırbey Fault, located to the east of the Erciş fault and extending roughly in the northeast–southwest direction, may be active.

The Eastern Anatolian plateau, one of the youngest and widest plateaus of the world, represents a suture zone at which the northern and southern branches of Neotethys come together 9,10 . The basement of the Eastern Anatolian plateau consists of micro-continents stacked together between Late Cretaceous-Early Tertiary and separated from each other by ophiolite belts and accretion complexes 1,62 . Five different tectonic blocks are recognized in Eastern Anatolia: the Eastern Rhodope-Pontide fragment, the Northwest Iranian fragment, the Bitlis-Pötürge unit, Autochthonous units of the Arabian continent and the Eastern Anatolian Accretionary Complex. Except for the Eastern Anatolian Accumulation Complex, all other tectonic blocks correspond to the microcontinents mentioned above 29 . The Eastern Anatolian Accretionary Complex (EAAC) represents the remnant of a huge subduction accretion complex located between the Rhodop-Pontide and the Bitlis-Pötürge micro-continent, formed within Late Cretaceous-Oligocene 26 .
The collision between the Eurasian and Arabian continents occurred in Serravallian (~ 13-11 Ma) 9,10,29 . Volcanic activity started right after the rapid block uplift of Eastern Anatolia and produced different volcanic products that spread throughout the region (Fig. 2) 27,29,47,63,64 . Volcanic activity first started around Erzurum-Kars plateau with calc-alkaline lavas in the north, then migrated to the south-southeast and became more alkaline 28,29 . This volcanic activity has produced a large volume of volcanic material that covers almost two-thirds of the area and in some places exceeds 1 km in thickness. Besides fissure eruptions in volcanic activity, there are also many volcanic centers (e.g. Ağrı, Nemrut and Tendürek Mountains).
The first stage of volcanism in Middle Miocene produced Aladağ Volcanics, which are widespread to the north-northeast of Lake Van 42,65,66 . The next stage of volcanism started at the beginning of Late Miocene (~ 10 Ma). Located in the area between the Ilıca and Deliçay rivers, these rocks have a widely varying chemical composition from basalts and sparsely trachybasalts to dacites 42 . Pliocene volcanism started 5.8-3.7 Ma as trachytic ignimbrites and tuffs that cover a vast area northwest of the town of Erciş. The activity of the Etrüsk Volcano up to trachydacites, trachytes and trachyandesites is the last phase of Pliocene magmatic activity. The activity of this volcano covers a time period between 4.3 and 3.7 Ma [42][43][44] . Quaternary volcanism in the north  82,83  We collected paleomagnetic oriented samples between 2015 and 2017 from the volcanic rocks whose ages and geochemical compositions have already been known from radiometric aging and petrography methods 42,43 to determine the tectonic evolution of the North of Lake Van. The orientations of the samples were determined using magnetic/sun compasses and samples were drilled using a portable oil-powered motorized drill with water-cooling, diamond-coated, non-magnetic drill bits. The paleomagnetic samples were collected around the northeast of Lake Van from Pleistocene volcanics (38 sites), Pliocene volcanics (82 sites), and Late Miocene volcanics (14 sites). To increase the number of samples at paleomagnetic sites and make them statistically more . Geological map of the study area including paleomagnetic sample sites of our study. Map were re-arranged from 85  www.nature.com/scientificreports/ reliable, we have combined two or more very close sites from the same age and rock type to create sites with a larger sample count. Thus, we have 32 Pleistocene, 58 Pliocene and 10 Late Miocene sites (Fig. 2).

Laboratory studies
The paleomagnetic laboratory studies were performed at the "KANTEK Paleomagnetism Laboratory" which is a collaborative laboratory of Boğaziçi University and Istanbul Technical University (ITU). Cylindrical core samples were cut into standard paleomagnetic specimens (2246 in total) and were subjected to stepwise thermal demagnetization using "Magnetic Measurements MMTD-60" thermal demagnetizer. The thermal demagnetization is applied in increments of 25-50 °C up to a maximum temperature of 650 °C. Molspin spinner magnetometer was used to measure magnetization directions and intensities of the Natural Remanent Magnetization (NRM) after each step of thermal demagnetization. Principal component analysis 67 and orthogonal vector diagrams 68 were used to describe the Characteristic Remanent Magnetization (ChRM). The ChRM directions and their statistical parameters for each site were determined using standard Fisher statistical analysis with 45° cut-off 69 . Errors in declination (ΔDx) and inclination (ΔIx) were calculated from the A95 of the Virtual Geomagnetic Pole (VGP) distribution for all sites. The criteria defined by Deenen et al. 70,71 indicate that the determined A95 value of the VGP distribution needs to be between the N-dependent values of A95min and A95max to represent the paleosecular variations (PSV) in the geomagnetic field. Remasoft 3.0 Paleomagnetic Data Browser and Analyzer software have been used to interpret the demagnetization diagrams. The reversal test and its classification developed by McFadden and McElhinny 72 were used to determine whether the two distributions with the positive and negative polarity means have a common mean direction. Rock magnetic studies (Isothermal remanent magnetization-IRM and high-temperature susceptibility-HTS) have been carried out to determine the magnetic minerals responsible for permanent magnetization and to determine the change with the coercive force in addition to paleomagnetism studies. All measurements of rock magnetic studies were done in Yılmaz İspir Paleomagnetism Laboratory at Istanbul University-Cerrahpaşa, Department of Geophysics.
HTS measurements have been carried out on 16 representative samples by heating in room conditions. The heating and cooling phases of the grinded sample between room temperature (24 °C) and 650 °C were done using the Bartington MS2 Susceptibility/Temperature System with Bartington MS2 susceptibility meter. IRM studies were performed on pilot samples from each rock type to detect the minerals responsible for the magnetization in the rock.

Analysis results
Rock magnetism studies. We made at least one HTS measurement for each rock type to determine the rocks' magnetic properties and to identify its magnetic behavior at different temperatures. The analysis of HTS measurements performed in 16 samples of different age and rock types show three different types of behavior. The red curves given in Fig. 3 illustrate the heating phase and the blue curves indicate the cooling phase.
Low Curie temperatures are observed between 150 and 250 °C in one of the sample groups (cay1, colp, san2, koz5, inc1), which is an indication of presence of titanomagnetite component in these samples.
Some of the samples show reversible behavior (atok, blk, cay1, inc1, koz5, ykoz5, ykr) with no remarkable difference between heating and cooling curves. On the other hand, the heating and cooling curves of some samples differ considerably, in relation to remarkable degree of alteration (ack, kad2, cay4, colp, yayla2). Heating curves depict a reduction of around 400 °C in two samples (ack, haci), which indicates the transformation to maghemite or the existence of Ti-rich titanomagnetite.
IRM measurements were applied to the samples taken from 33 sites representing each of the volcanic rocks located to the north of Lake Van. IRM curves in Fig. 3 indicate that magnetite is the responsible mineral for magnetization. Two different types of behavior are observed in the IRM acquisition curves. The first type of IRM curve is characterized by low to moderate coercivity phases, reaching saturation between 0.1 and 0.3 T. There are 21 sites (Arg, Bend, Colp, Inc1, Orn, etc.) where the saturation is observed in IRM curves and magnetite mineral is dominantly responsible for their magnetization. The second type of IRM curve is characterized by a rapid increase of magnetization in low fields at first (up to 1 T) and then a small increase without complete saturation at 1 T (Fig. 4). There are 12 sites (Ykoz, Ykr, Haci, Skr, Tprk4, etc.) whose IRM curves are classified as the second type, indicating that both magnetite and hematite are responsible for the magnetization. Hematite was not dominant in any of the samples.
Paleomagnetic studies. In the study area, there are volcanics of different rock types in the Late Miocene-Pleistocene range. During the interpretation of the paleomagnetic data, we divided these volcanics into 3 different age groups as Late Miocene, Pliocene and Pleistocene. Late Miocene volcanism in the region is represented by products from Aladağlar and Meydan Mountains. These volcanics are located to the north of the Zilan Valley and Etrüsk Volcano within the vicinity of the Tendürek Mountain (N-NW of Erciş Village). Late Miocene-aged sites are generally located in the north-northeast of Erciş district. The Pliocene-aged volcanics, which are the most common rocks in the study area, are around Van, Erciş, Etrüsk Mountain and Muradiye. Pleistocene volcanics are located in the north of the Erciş district and the west of Etrüsk Mountain as Girekol, Yüksektepe and Karnıyarık volcanics. Pleistocene aged sites are also located in these regions.
Group mean directions. Pleistocene aged rocks in the north of Lake Van have been sampled from 32 paleomagnetic sites. Except for 11 statistically unreliable paleomagnetic sites which are excluded from evaluation, all sites have normal polarities (Table 1) Fig. 4b). Four sites have normal and 12 sites have reverse polarity.
The group near the Muradiye is named Pliocene_EM and has six sites. The mean direction of this group is calculated as D = 5.9° and I = 43.8° with statistical parameters N = 6, k = 38.84 and α 95 = 10.9° before tilt correction, D = 8.3° and I = 46.2° with statistical parameters N = 6, k = 38.84 and α 95 = 10.9° after tilt correction. One site has normal and five sites have reverse polarity (Table 1; Fig. 4b).
Late Miocene aged rocks in the north of Lake Van were sampled from 10 paleomagnetic sites. There is not any statistically unreliable paleomagnetic sites (Table 1). A total of four sites showing suspicious directions diverging from the average distribution and sites with A95 values outside the envelope of A95min and A95max were not taken into evaluation.
Paleosecular variation. Paleosecular variation (PSV) should be averaged in paleomagnetic studies, so that the paleomagnetic directions show only tectonic movement 70 . PSV should be considered unreliable if A95 values are above or below the A95min and A95max limits 70 . In this study VGPs were calculated from paleomagnetic rotations for each site and group. A95 values for all of the groups and the most of the individual samples fall within the A95min/A95max confidence envelope; samples otherwise were not included within the evaluation. Therefore, it is plausible to consider that PSV is adequately averaged in our paleomagnetic data set.

Discussion
The Pleistocene pole position is calculated as 77.4° N, 278.3° E (dp = 4.7, dm = 3.2, α 95 = 3.5°) showing counterclockwise rotation of R ± ΔR = 13.4° ± 3.8° when compared with the expected stable Eurasia reference pole (λ ref , / Φ ref = − 88.5°/353.9°, α 95 = 1.9°) of 73 using PMGSC (version 4.2) software 74 (Fig. 7a). Examining the rotations in the southern part of the Erciş Fault (group Pliocene_SW) implies a counterclockwise rotation: R ± ΔR = 14.5° ± 6.1° (Fig. 7b). On the other hand, no significant rotation (R ± ΔR = 0.6° ± 7.4°) is observed on the northern side of Erciş Fault (group Pliocene_NE) (Fig. 7b). Also, the remarkable clockwise rotation R ± ΔR = 24.4° ± 17.0° to the north of Van (group Pliocene_EV) and clockwise rotation R ± ΔR = 6.9° ± 9.4° of the sites near Muradiye indicate that the prevailing tectonic movement of the region is clockwise (Fig. 7b). In summary, Pliocene paleomagnetic results reveal that there are a lot of remarkable rotation differences around the Erciş Fault, most of which are caused by the Erciş Fault itself. The results from Late Miocene volcanic rocks indicate that there is almost no www.nature.com/scientificreports/ significant rotation (R ± ΔR = 1.8° ± 13.7°) in the region (Fig. 7c). In Table 2, net rotation amounts of other ages and location groups are given respectively. These values were used in the tectonic interpretation of paleomagnetic data obtained in this study. The difference between the observed poles (λ obs , ϕ obs ) and the reference poles and expected tectonic rotations (R) were computed using the pole-space method of Beck 75 and the 95% confidence limits (ΔR) were determined after Demarest 76 . Hisarlı et al. 16 named the VB as a region that includes the Lake Van and its surroundings, from the Karlıova Triple Junction to the east. According to the paleomagnetic data obtained from this study, VB must have been rotating clockwise starting from Late Miocene.
Our study area does not coincide with the area of the other relevant studies 16,45,46 and covers a relatively narrower area compared to them, allowing the investigation of smaller "micro-block" movements instead of mono-block movements. Our study also includes more paleomagnetic samples and distribution in the north of Lake Van and results show both clockwise and counterclockwise rotations in the area which can be interpreted that the Van Block is divided into micro-blocks that have different rotation directions.
If the lineament (i.e. fault) between the blocks of same aged rocks determined by the paleomagnetic rotations coincide with the lineament observed in the active fault map, it can be said that the slips along the faults by the current earthquakes are the continuations of the past tectonic movements.
According to the paleomagnetic rotations of the Pleistocene volcanic rocks, it can be concluded that the whole region rotated counterclockwise (~ 13.4° ± 3.8°) and moved as a mono-block. Selçuk et al. 77 claimed that the slip rate of the Çaldıran Fault was ~ 3 mm/year (for 290,000 years) and that the maximum slip was ~ 900 m. The rotation resulting from such a small slip cannot be measured with paleomagnetic data. For this reason, it www.nature.com/scientificreports/ is not relevant to expect a clockwise rotation in the area between the Çaldıran Fault and the Erciş Fault, two of which are dextral strike-slip faults. Therefore, it is clear from the obtained data that the region has rotated counterclockwise (~ 13.4° ± 3.8°) since Pleistocene. Rotations from Pliocene rocks are given in Fig. 7.a. To determine the rotations in the Pliocene-Pleistocene time interval, counterclockwise rotation of the Pleistocene rocks (~ 13.4° ± 3.8°) is needed to be rotated clockwise. After this period, Pliocene_NE rocks were rotated ~ 13° clockwise and Late Miocene rocks were rotated ~ 11° clockwise in the Late Miocene-Pleistocene period. It is observed that the Çakırbey Fault is formed before the Erciş Fault (Pliocene) and causes a rotation of ~ 37.8° towards the east of the Erciş Fault. This suggests that the region was subjected to an active rotation in the counterclockwise direction after Late Miocene until the Quaternary.
Emre et al. 78,79 does not mark an active fault in the northwest of Lake Erçek. However, according to our results, we suggest that the Çakırbey Fault, which is roughly oriented northeast-southwest towards Erciş Fault, must be active (Fig. 7) and thus different rotations may have developed on both sides of this fault.
Copley and Jackson 80 , investigating the active tectonics of the Turkish-Iranian plateau, examined the right lateral strike-slip faults and claimed that the parallel striking Erciş Fault and Çaldıran Fault are 11 km and 1.3 km offset, respectively. Furthermore, they stated that the combined slip rate for these faults was about 8 mm/year and that a time of about 1.5 Ma would be necessary to generate the combined 12.3 km shift at this speed. Authors stated that the removal of 11 km of dextral movement along the Erciş Fault restores the mountain-fronts (Fig. 8a) and the edge of volcanic rocks (Fig. 8b).
Koçyiğit 81 claimed that the Erciş Fault cut across Early-Middle Miocene marine sequence and Quaternary volcanic rocks to sedimentary packages, also Çakırbey Fault cuts across the Early-Middle Miocene marine limestone, Quaternary volcanic rocks. However, as it is seen from our new paleomagnetic rotations, the northern part of the older Çakırbey Fault was cut and shifted by Erciş Fault. This displacement caused the oppositely directed rotations around Muradiye. Table 2. Group mean ChRM directions (D, I, α 95 ) after tectonic correction and expected tectonic rotations (R) with respect to the stable Eurasia reference poles 73 . The reference pole (λref /Φref = 88.5°/ 173.9°, α 95 = 1.9°) for the stable Eurasia is obtained after 73 . Here α 95 is the statistical parameters after 69 . R is the angle of vertical axis rotation (positive indicates clockwise rotation) with respect to the direction computed from the stable Eurasia paleomagnetic pole with 95% confidence limit ΔR (after 75 ).  Fig. 9, the focal mechanism distributions of M > 4.5 earthquakes on the possible Çakırbey Fault are given on the northeastern border of Lake Van. Also, Koçyiğit 81 shows that the Eastern plateau is under the influence of the N-S direction of the Arabian plate by using GPS data and focal mechanism solutions of different earthquakes. Our results and the distribution and the focal mechanisms of earthquakes are examined, it is clear that a left lateral strike slip Çakırbey fault exists and is active.
On the northern southern parts of the Erciş Fault, ~ 13° clockwise and ~ 1° counterclockwise rotations are seen, respectively, between Pliocene-Pleistocene time intervals. These rotations indicate a deformation around a right lateral strike-slip fault and the block movements on both sides of the fault. In summary, according to the results obtained from this study, the study area had a rotation of ~ 2° counterclockwise in the Late Miocene-Pliocene time interval. It was rotated clockwise ~ 13° in the Pliocene-Pleistocene period. Since the Pleistocene, it has rotated ~ 14° counterclockwise.

Conclusions
Widespread and intense seismic activity, rapid uplift of the Eastern Anatolian plateau, westward movement of the Anatolian plate along the NAFZ and EAFZ, strike-slip fault zones in the region and Neogene magmatic activity throughout the region are indicators of the region's complex tectonic structure. At the same time, the fact that the paleomagnetic rotations obtained from our study using the same aged rock groups scattered in different regions is another indicator of the high tectonic activity of the region. In this study, 62 reliable paleomagnetic sites were taken from the Late Miocene to Quaternary volcanic rocks located to the north of Lake Van, Eastern Anatolia, in order to examine the tectonic deformation of the north of the Lake Van from a paleomagnetic aspect. Hisarlı et al. 16 stated that the region moved as a single block rotating clockwise by using a limited number of paleomagnetic sites sparsely covering the study area. By increasing the number and distribution of paleomagnetic sites, our study revealed smaller blocks that are moving in different directions, both clockwise and counter clockwise in the area.
When the inclination angles of the group mean values given in Table 2 are considered, the value is very close (with 2-3° difference) to the expected inclination angle value (58°) for the region, which is a sign that the region has not been making a latitudinal movement since the Late Miocene.
In the vicinity of the western part of Erciş Fault, rotations are observed in all age groups in the Late Miocene-Pleistocene time interval depending on the activity of the Erciş fault. Also, paleomagnetic data from the Pliocene volcanics on the north-western part of Erçek Lake, Van-Muradiye highway (around Çolpan village near the Lake Van) defined R ± ΔR = 24.4 ± 17.0 clockwise rotation. However, there is no active fault in this area on the new active fault map of Turkey 78,79 . These paleomagnetic rotations and focal mechanism solutions of the recent earthquakes in the field indicate that the former Çakırbey Fault might still be active.